The dew point is the temperature at which air with a given moisture content becomes saturated. This measurement is accomplished by artificially lowering the temperature of a surface and then noting the temperature at which moisture first condenses.
Many types of dew point sensors are known to the art. Some dew point sensors are capacitive and resistive devices and employ moisture-sensitive material to sense condensation. The moisture-sensitive material is formed into a condensation surface on the dew point sensor, and heat is slowly removed from the sensor until condensation begins. The moisture-sensitive material reacts to the presence of the moisture of condensation, for example by changing resistance. The temperature of the sensor is continuously monitored, and when this change of resistance is detected the temperature is recorded.
Another prior art form of dew point sensor is a condensing mirror type provided with a reflective surface. When the temperature of the reflective surface is reduced to the dew point temperature, condensation takes place. The moisture of condensation is detected either by visual inspection or by the use of a photocell or other light-sensitive device that responds to the change in reflectivity caused by the presence of moisture.
Unfortunately, none of the dew point measuring instruments just described is able adequately to satisfy the requirements for use in industrial processes. The high temperatures that occur in industrial processes damage the components used in many sensors, and the contaminants often found in such processes alter the absorption properties of many moisture-sensitive sensors and cloud the reflecting surfaces of reflector type sensors. Moreover, those devices have a limited range of operation and are limited to temperatures below 200.degree. F. Furthermore, capacitive and resistive devices provide indirect dew point measurements. Thus, these instruments must be calibrated for specific relative humidities from which the dew point temperature is calculated.
Various other methods of achieving accurate measurements under industrial process conditions have been attempted. One such device is disclosed in Rall et al., U.S. Pat. No. 4,579,462, issued Apr. 1, 1986, which describes a dew point measuring apparatus that is characterized by a heat flow sensor which, in one embodiment, comprises a condensation surface and an embedded differential thermopile. The heat flow sensor measures the flow of heat into the surface rather than the temperature of the surface. It senses the exothermic release or endothermic absorption of heat as moisture condenses on or evaporates from the surface at the dew point temperature. When the apparatus is used to determine the relative humidity of a gaseous atmosphere, the temperature of the sensor is slowly lowered until the dew point of the atmosphere is reached. As soon as condensation begins, heat of condensation is released from the condensate and flows into the condensation surface. The heat flow sensor responds to this heat flow, thereby permitting the determination of the atmosphere's dew point, and hence of its relative humidity.
In practice, the above described apparatus requires a pair of heat flow sensors mounted adjacent a heat sink. A coolant is circulated through the heat sink to slowly reduce the temperature of the sensors. A heater is provided which is actuable to raise the temperature of the sensors. By actuating and deactuating the heater the temperature of the sensors can be cycled up and down through the dew point of a gas or atmosphere under test, thereby permitting the atmosphere's dew point to be tracked. One of the sensors is kept at a slightly higher temperature than the other, and the output signals from the two sensors are compared, as by connection in series opposition. A stream of gas under test is passed over the sensors. A relatively sudden change or imbalance in the signal from the oppositely electrically connected sensors occurs as moisture condenses on the cooler sensor and the exothermic heat of condensation is released. The surface temperature of the cooler sensor at that instant is the dew point of the gas.
Although the dual heat flow sensor device is better suited than capacitive and resistive devices for measuring gases with contaminants, it nevertheless has limitations. Its principal drawback is that with dual heat flow sensors, it is difficult to calibrate over wide dew point temperature and relative humidity ranges.